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Worcester Greenstar E9 Fault

When your Worcester Greenstar displays an E9 Fault it means it has overheated. The safety limit thermostat has stopped the boiler as it is unsafe to keep working.

worcester greenstar E9 fault

What does an E9 fault mean?

An E9 fault means the boiler has detected a temperature of above 105oC and is locked out for safety.

So this means either:

  • The boiler has actually gone to a temperature above 105oC or:
  • The temperature sensor is faulty so the boiler thinks it is above 105oC.

What causes an E9 fault?

  • Low system pressure.
  • Dry system
  • Faulty circulating pump.
  • Blocked pipework.
  • Blocked main heat exchanger.
  • Faulty temperature sensor.
  • Isolating valves turned off.
  • Air in the central heating system pipework.

Low system pressure

This is normally easily fixed by using the filling loop to top up the pressure. An occasional pressure drop is common. More regular pressure drops should be investigated and repaired.

Dry system

Similar to low pressure, a dry system is a lack of water in the boiler. This is normally caused by a problem with the feed tank in the loft.

An open central heating system is supplied from a small water tank/cistern at a high point in your property.

An water loss from leaks, draining or bleeding the radiators is replaced by the ball/float valve in this tank.

The main reasons the water isn’t replaced are:

  • Faulty ball/float valve. The valve hasn’t opened when the water level in the tank has dropped.
  • Blocked cold feed. The pipe supplying water to the system from the tank is blocked with sludge. There is no water flowing to refill the system if there is a water leak or if radiators are bled.
  • Ball/float valve isolated. This surprisingly common problem can take years to show itself. When work is done on an open system the ball valve is normally isolated. It’s very easy to forget to reinstate the water supply. It’s also an easy shortcut by an unscrupulous plumber to turn off a ball valve and leave it. This will stop an overflow pipe running but leave a customer with a different problem several months/years later.

Faulty circulating pump

The heat generated inside the boiler is moved by the circulating pump. If a pump has failed or is beginning to fail the heat stays inside the boiler causing an E9 fault.

The pump should be replaced by a gas safe engineer.

Blocked pipework.

Sludge and dirt from the heating system can build up over time causing restrictions in the pipe work. This reduces the amount of heat the circulating water can remove from the boiler when it is running.

If the circulation is reduced too much the boiler will overheat. This is especially likely with powerful boilers on smaller heating systems.

A more common fault for blocked pipework would be an unusual temperature difference between flow and return.

Blocked pipework can be fixed in the following ways:

  • Cleaning chemicals injected into the system then drained and removed. This only works on very lightly sludged systems.
  • Cleaning chemicals followed by a flush using the mains cold water or powerflushing machine. This works on the majority of systems. If all the radiators are still heating this method is normally enough.
  • Powerflush with very aggressive chemicals. This is the method that gets through the worst blockages and heaviest sludge. A full powerflush of a system can take several days and even then can’t guarantee total success.
  • Cut out and replace blocked pipework. This is normally reserved for smaller diameter (10mm and 8mm) microbore pipework. Although it is more expensive and intrusive it will solve most circulation problems.

Blocked main heat exchanger

Over time the main heat exchanger can become blocked with sludge and dirt. This reduces the amount of heat that can be removed by the circulating water causing overheating.

This will normally show itself by noises when the boiler is in use that sound like a kettle boiling.

The sludge will eventually build up to the point where the water can’t transfer the heat out fast enough. This is when you will get E9 faults.

A blocked main heat exchanger can be fixed in the following ways:

  • Add cleaning chemicals to the system. The sludge and dirt that is causing the blockage will be distributed throughout the system. Cleaning the system will remove the sludge and improve circulation. (See above for more details).
  • Add “boiler silencer” chemicals. These chemicals are designed to remove the dirt and sludge that has accumulated on the hot surfaces in the boiler. It doesn’t remove the dirt from the system. The dirt is relocated to somewhere else (normally the bottom of a radiator) where it does less harm.
  • Replace the main heat exchanger. This is a very expensive repair for parts and labour. A new heat exchanger can be a similar price to a replacement boiler so should be taken lightly.

Faulty temperature sensor or PCB.

A faulty temperature sensor can give the boiler a signal that it is overheating when it is not. The boiler will have a set of control temperature sensors and an overheat sensor.

Boiler temperature sensors come in two types:

  • Control sensors. The resistance of the sensor changes depending on the temperature. The change in resistance follows a graph that the boiler understands and expects. So a low temperature will give a set resistance and a high temperature gives a different set resistance. If the sensor fails the reading may tell the boiler to continue heating up when it should be cooling down.
  • Overheat sensors. The overheat sensor will be a closed switch when the temperature is below the maximum expected temperature. This will allow electricity to flow and the boiler to operate normally. If the boiler goes above the expected temperature the switch opens and stops electricity flowing.

Faulty control sensors normally show themselves by strange behaviors. For example radiators or hot water being cooler or much hotter than expected.

A faulty PCB can show itself by the numbers on the display jumping around in a unusual way.

Faulty sensors or PCB have to be replaced by a gas safe engineer. They can be tested using a multimeter to make sure they are in the range set by the boiler manufacturer.

Isolating valves turned off

This is normally just an error by a customer or heating engineer. The following valves can be isolated (normally by accident).

  • The valves under the boiler should not be touched by the customer. They are also prone to leaking so most engineers would be wary of turning them off.
  • The valves on a magnetic filter are closed during a service. It is very easy to forget to turn them back on again.
  • Radiator valves can be turned off for other work to be done then forgotten about. Don’t assume they are open because sometimes they aren’t.

Air in the central heating system pipework.

Air can get trapped in pipework in a way that the force of the circulating pump can’t move. If the pump can’t move the water around the pipework the boiler will overheat giving the E9 fault.

This fault is normally after the heating system has been fully or partially drained.

How do I clear air from my central heating system pipework?

  • Drain and refill the system.
  • Back filling from the water mains. A connection is made between the heating system and the mains pipework. Mains pressure water is then used to push the air out of the pipework.
  • Turn off all radiator valves except one. This make the pump work against a single set of pipework. Hopefully the extra water flow can push the air out of the pipework.
  • If you have a hot water cylinder run the hot water circuit until the water starts to circulate. Most cylinders have an air release on the pipework that can be used to bleed air out. Once the hot water circuit is working it is normally easier to get the heating going.
  • Be more patient. No one likes this plan but it is quite effective. Some of the worst air locks I’ve ever dealt with have cleared themselves after a period of time. My technique was to turn the boiler thermostat as low as possible then run the heating over night. The radiator won’t heat up if the boiler stat is low. However the pump will push against the air lock and eventually move it.
  • Powerflushing is the last resort. If the system is really air locked a powerflushing machine can be used. The powerflush circulates water at great speed to force the air out of the pipework.

Can I fix an E9 Fault myself?

Anything inside the boiler should only be touched by a gas safe engineer. It can be very dangerous and may be illegal to work on a gas appliance if you aren’t qualified.

You can however repair anything outside the boiler. Draining, refilling, back filling etc are all fine to do if you are confident and competent.

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